The present invention relates to optical scanners and more particularly to a multiple facet holographic disk scanner having parameters which are controlled, at least in part, on a real time basis as a function of facet diffraction efficiency.
Optical scanners of the type commonly employed in supermarkets include a laser light source and an optical system which re-directs light from the source along predetermined scan lines to produce a scan pattern above the scanner suitable for detecting bar coded labels on products passing over the scanner. The optical system typically includes a rotating component in the path of the laser beam for causing the laser beam to be deflected along arcuate scan lines, which may be folded or further shaped by a set of fixed mirrors to produce the actual scan pattern.
In one known type of scanner, the rotating component is a glass disk which carries a number of sector-shaped holographic facets. Each of the facets may be characterized as an optically generated diffraction grating which bends or diffracts an impinging laser beam along a predetermined path. Each of the facets may be constructed using conventional holographic techniques by aiming two beams of coherent light at an unexposed sheet of photosensitive material, such as silver halide film or dichromated gelatin film. Where the two beams overlap, interference patterns are recorded in the photosensitive film. The films are developed by similarly conventional techniques suitable for the particular film material employed.
Multiple facet holographic disks have advantages over rotating mirror wheels of the type commonly known in the prior art. The holographic disks are considered to be cheaper and easier to build. Also, a scan pattern can be changed without significant difficulty simply by changing one or more facets on the disk.
While multiple facet holographic disks have these and other advantages, they also have certain drawbacks. Because of the nature of the optical processes by which the facets are made, some facets are more efficient at diffracting or bending impinging light rays than other facets. For example, one facet may bend or diffract 65% of the impinging light, another facet may diffract only 45% of the impinging light. In both of these cases, most of the remaining light will continue on through the holographic facet in a straight line path, with a small portion being scattered, absorbed or reflected by the holographic medium or its substrate. Conventional lasers are fixed power devices which provide an output beam having substantially a constant power level. Changes in diffraction efficiency from facet to facet produce changes in the power level of the diffracted beam from facet to facet.
Since the amount of light returned from an object is at least partially dependent on the power level of the light initially impinging on the object, the power level of returned signals can fluctuate widely due to facet to facet changes in diffraction efficiency. If the returned signal fluctuates too much, it will be distorted by electronic signal processing circuitry needed to detect and decode labels on the products being scanned. The distortion may prevent the label from being successfuly read and, in some instances, may cause the label to be incorrectly read.
While the preceding material addresses only facet to facet variations in diffraction efficiency, the fact is that diffraction efficiency also varies within a facet as well. Changes in diffraction efficiency within a facet cause the same sorts of problems as facet to facet changes.
The problems caused by nonuniform diffraction efficiencies in holographic scanner disks have been recognized. Various attempts have been made to solve these problems. In one known type of scanner, the gain factor for the electronic signal processing circuitry is set at a level dependent upon an average diffraction efficiency value obtained by statistically sampling a number of holographic disks to be used in the scanners. Obviously, this approach does not consider variations in diffraction efficiency from disk to disk, from facet to facet within a disk, or within individual facets.
Another proposed partial solution to the problem of nonuniform diffraction efficiency calls for the placement of an auxiliary track on each holographic disk. The track which might be read by a separate optical read head or by a magnetic head could include diffraction efficiency values for specific facets on the holographic disk. The auxiliary track would not include any diffraction efficiency information for most of the facets on the disk. Moreover, the proposal does not provide any suggestion for solving the problem of variations in diffraction efficiency within a given facet.
Another approach to the problem of nonuniform diffraction efficiency calls for the gain of the signal processing circuitry to be adjusted to maximize the percentage of labels which are correctly read on the first pass. The gain adjustment is indirectly influenced by disk diffraction efficiency but does not actually provide compensation for facet to facet variations or intra-facet variations in diffraction efficiency.